This invention relates generally to coupling mechanisms for engines and, more particularly, to fixed curvic couplings used in the assembly of components that comprise rotating elements, such as impellers, turbine wheels and couplings for gas turbine engines and other types of engines having highly stressed components.
Curvic couplings for use in the assembly of rotating element components were developed to meet the need for permanent coupling mechanisms requiring extreme accuracy, maximum load capacity, and relatively economical rates of production. The curvic design provides an accurate, light, compact, and self-contained connection in which the curvic teeth serve as centering and driving devices. The most widely used type of curvic coupling used in gas turbine engines is the fixed curvic coupling. The fixed curvic coupling can be described as a precision face spline with teeth that possess a high degree of accuracy of tooth spacing, fine surface finish, and precision axial location.
The predominant application of fixed curvic couplings is in the assembly of the elements that comprise a gas turbine engine rotating group, i.e., impellers, turbine wheels, and couplings. Many standard turboprop and turbofan engines are examples of this application, in which the disk-like members of the rotating group are mounted solely on fixed curvic coupling teeth. A shouldered shaft acts merely as a tie-bolt and does not interfere with the centering action function of the curvic teeth.
The design of the curvic coupling itself is a function of several variables. Some of these variables include the amount of torque required to be transmitted by the coupling, the shear load and bearing load limits of the materials used in the mating designs, and the amount of tiebolt tension required to maintain full xe2x80x9cstackxe2x80x9d engagement under all operating conditions. Many times, the curvic teeth are located near the inside diameter (bore) of the turbine or compressor disks, which are generally regions of high hoop stress. In order to maintain acceptable curvic root stress levels, it is important to position the curvic teeth axially and radially so that the curvic teeth background stress levels are maintained at acceptable levels. Frequently, engine manufacturers employ curvic coupling designs that vary depending on the specific application. Designs vary geometrically in all major features, including: outside diameter, number of teeth, pressure angle, profile curvature, tooth depth, etc.
The curvic design is highly influenced by manufacturing constraints. Tight gaps are typically maintained between curvic teeth. These tight gaps are required to keep the xe2x80x9cstack"" dimensions short, positively impacting overall engine length (weight), shaft dynamics, and other important engine considerations. Sometimes the gaps between curvic teeth are used to meter cooling air required to maintain acceptable cool turbine disk cavities.
Care must be taken to ensure that the curvic root fillet is designed to avoid manufacturing issues such as (1) root fillet radius interference with the mating tooth, (2) mutilation of the opposite root fillet radius by the grinding wheel, or (3) the creation of xe2x80x9cfinsxe2x80x9d, or residual material in the center of the gable bottom. Proper design of both the curvic root (fillet radius and gable), along with the design of the grinding wheel itself, are crucial to ensure curvic tooth manufacturability. The traditional approach is to use a simple single fillet that is sufficiently small so as to allow ease-of-manufacturing.
The small, single fillet often results in a stress concentration feature that limits the overall fatigue life of the component. As previously stated, the position of the curvic tooth on the disk is often near a region of high background hoop stress, which is a result of the high rotational speeds of the disk as well as thermal gradients formed within the disk during engine operation. Additional curvic root stresses are the result of the axial compressive load, which keeps the xe2x80x9cstackxe2x80x9d components together, and the torque loads which are carried through the curvic coupling. Recent three dimensional finite element modeling has shown that some disk curvic root fillet stress levels are higher than originally thought, and the resulting service lives do not meet design intent. Expensive and time-consuming redesign of the disk is one method for reducing these limiting curvic root fillet regions. The redesign may include a material change and/or geometric changes that require replacement of not only the subject component, but also several mating components. Design, substantiation, and certification time and cost for this type of redesign may be prohibitive.
The problem of high stress levels and service life limits on curvic couplings and geometrically similar gear applications has been addressed in the past. Of particular interest in this regard are the following references and examples:
U.S. Pat. No. 6,065,898 to Hale involves a three tooth kinematic coupling, which is distinctly different in geometry and application from curvic couplings. However, as is generally the case in coupling designs, there is considerable emphasis given to a design, which reduces localized stress points, and increases stiffness and load capacity.
U.S. Pat. No. 6,164,880 to Pulley, et al, discloses a method for producing a specifically shaped fillet on a gear. The invention focuses on reduction of the bending stress as a means of reducing the likelihood of gear tooth breakage, but does not address the background hoop stress associated with high energy rotating components, nor does it address compound root radii and compound fillets.
The specific problem of curvic coupling hoop stresses and geometries was addresses by A. Tang of the Textron/Lycoming Company in technical memorandum number B10-GLC38-006-90 dated Apr. 27, 1990. The Textron/Lycoming Company subsequently merged with the assignee of this invention. The study documented in said memorandum was accomplished to define a curvic coupling geometry, which would reduce hoop stresses to acceptable levels. Results showed that a larger fillet radius with a smaller air space gap would reduce hoop stresses to acceptable levels. Although generally similar to the present invention, the concept proposed by Tang was not reduced to practice and did not address use of a compound fillet radius as conceived in this case the means to control hoop stresses.
Thus, there is a need for a redesigned curvic coupling, which will reduce stress values to acceptable levels without exceeding space limitation constraints with the ultimate result being a significant increase in service life.
In one aspect of the present invention, a toothed coupling mechanism for assembly of rotating elements of gas turbine and other engines to a shaft, comprises a pair of coupling halves, a plurality of concentric spline teeth for transmitting torque, the spline teeth having a compound fillet radius consisting of both a large radius positioned in the root area of each tooth and small radius positioned between the larger radius and the pressure surface of each tooth, the combination resulting in a lower fillet peak stress value.
In another aspect of the present invention, a method is disclosed for reducing the stresses and increasing the service life of toothed coupling mechanisms for assembly of rotating elements of gas turbine and other engines to a shaft. The method involves the steps of positioning a large filet radius in the root area of splined teeth on the coupling halves of the coupling mechanism, positioning a small filet radius in the pressure surface of each tooth, and connecting the large filet radius to the small filet radius forming a compound filet radius resulting in lower fillet peak stresses.
Other aspects, advantages and features of the invention will become more apparent and better understood, as will equivalent structures, which are intended to be covered herein, with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims, and drawings.